1. Field of the Invention
The present invention generally relates to a mark (pattern) forming method for a moving body by using a laser beam with high accuracy and a moving body having a highly accurate mark; and in detail, relates to a rotating body having a mark for an image forming apparatus such as a photoconductor belt, a transfer belt, a paper carrying belt, a photoconductor drum, a transfer drum, and so on in an image forming apparatus such as a copying machine, a printer, and a facsimile. The present invention can be also applied to a positioning sensor and a pattern forming method.
2. Description of the Related Art
In an image forming apparatus which provides rotating bodies for forming an image such as a photoconductor belt and an intermediate transfer belt, in order to accurately align an image on a transfer material carried by a rotating movement unit of the rotating body, it is required that a moving amount and a moving position of the rotating movement unit of the rotating body be controlled with high accuracy. However, when the rotational speed of the rotating body is changed for some reason, the moving amount and the moving position of the rotating movement unit of the rotating body are also changed. Consequently, it is difficult to control a position difference of the image on the transfer material carried by the rotating movement unit with high accuracy.
Conventionally, in order to accurately control the position difference of the image caused by the moving speed change of the rotating movement unit of the rotating body, a rotary encoder is directly connected to the rotational axle of a driving roller of an endless belt type moving body such as a transfer belt and a paper carrying belt and to the rotation axle of a cylindrical member such as a photoconductor drum, and the rotational speed of a driving motor being driving means of the rotating body is controlled based on the rotational speed of the rotating body detected by the rotary encoder. Such an image forming apparatus is disclosed in Japanese Laid-Open Patent Application No. 6-175427. This image forming apparatus indirectly controls the moving amount (moving position) of the rotating movement unit of the rotating body by controlling the rotational speed of the rotating body.
In Japanese Laid-Open Patent Application No. 6-263281 (Patent Document 1) and Japanese Laid-Open Patent Application No. 9-114348 (Patent Document 2), a method is disclosed where marks are formed on a belt surface and the belt surface speed is calculated by a pulse interval obtained by detecting the marks with a sensor, and this calculated result is fed back to the control of the belt surface speed. According to this method, since the movement of the belt surface can be directly observed, the moving amount can be directly controlled.
The above conventional technologies do not specifically teach a method of forming marks on the belt surface, and do not make problems occurring during an actual use clear. As an actual example of the marks, it is considered that holes are formed as marks in the belt and are detected by a transmission type sensor. However, when the holes are formed, tensile strength of the hole forming parts is extremely decreased and stretching frequently occurs, compared with the other parts. Consequently, a correct belt carrying state cannot be obtained, a stress is concentrated on and a crack occurs in the hole forming parts, and there is danger of the belt breaking.
In addition, when the marks formed by the holes or reflection marks each formed by a metal reflection film are used, a leakage current occurs between the photoconductor body and the intermediate transfer belt to which high electric charge is applied. Therefore, a bad influence is given to the transfer process, and this may cause a breakdown of the apparatus.
A subject of the present invention is explained using a color image forming apparatus as an example.
First, referring to FIG. 1, a color image forming apparatus suitable to a case to which the present invention is applied is explained. This color image forming apparatus is a so-called tandem type apparatus in which plural electronic processing units 1K, 1M, 1Y, and 1C are arrayed in order from an upstream side of a moving direction (carrying direction) of a carrying belt 3, along the carrying belt 3 that carries a paper 2 to which an image is transferred as a recording medium. Each of these electronic processing units 1K, 1M, 1Y, and 1C, functions as an image forming unit. The electronic processing unit 1K forms a black image, the electronic processing unit 1M forms a magenta image, the electronic processing unit 1Y forms a yellow image, and the electronic processing unit 1C forms a cyan image. The internal structure is the same in each of them but the forming color images are different among them. Therefore, in the below explanation, structural elements of the electronic processing unit 1K for the black image are specifically explained, the specific explanations for the electronic processing units 1M, 1Y, and 1C are omitted, and elements with the signs M, Y, and C attached are only shown in the drawing.
The carrying belt 3 is an endless belt movably held by carrying rollers 4 and 5; one of them being a driving roller which drives the rotation and the other being a driven roller, and the carrying belt is rotated in an arrow direction by the rotations of the carrying rollers 4 and 5. A paper feeding tray 6 in which the paper 2 is stored is disposed under the carrying belt 3, and the paper 2 at the uppermost position being stored in the paper feeding tray 6 is sent out and is adhered to the carrying belt 3 by an electrostatic force at the time of image forming. The paper 2 adhered to the carrying belt 3 is carried to the first electronic processing unit 1K and a black image is transferred to the paper 2.
The above electronic processing unit 1K for the black image includes a photoconductor drum 7K being an image carrier; and a charger 8K, an exposing unit 9K, a developing unit 10K, and a photoconductor cleaner 11K disposed around the photoconductor drum 7K. A laser scanner is used as the exposing unit 9K, and the exposing unit 9K is structured so that a laser beam from a laser beam source is reflected by a polygon mirror and exposing light 12K is emitted via an optical system using an fθ lens, a deflection mirror, and so on.
When an image is formed, the circumferential surface of the photoconductor drum 7K is uniformly charged by the charger 8K in the dark, and then is exposed by the exposing light 12K (a laser beam in this example) from the exposing unit 9K corresponding to a black image, so that an electrostatic latent image is formed on the photoconductor drum 7K. This electrostatic latent image is changed to a visible image by black toner in the developing unit 10K, and a black toner image is formed on the photoconductor drum 7K.
This toner image is transferred onto the paper 2 by a transferring unit 13K at a so-called transferring position where the photoconductor drum 7K contacts the paper 2 on the carrying belt 3, and a single color (black) image is formed on the paper 2. Unnecessary toner remaining on the circumferential surface of the photoconductor drum 7K is removed by the photoconductor cleaner 11K, and the photoconductor drum 7K finishes the transfer and is prepared to form the next image.
The paper 2 on which the single color (black) is transferred by the electronic processing unit 1K is carried to the next electronic processing unit 1M by the carrying belt 3. In the electronic processing unit 1M, by the same process as that in the electronic processing unit 1K, a magenta toner image formed on the photoconductor drum 7M is transferred onto the paper 2 by registering the toner magenta image on the black toner image.
Further, the paper 2 is carried to the next electronic processing unit 1Y and by the same process a yellow toner image formed on the photoconductor drum 7Y is transferred onto the paper 2 by registering the yellow toner image onto the black and magenta toner images. By the same process, a cyan toner image is transferred onto the paper 2 by registering the cyan toner image onto the black, magenta, and yellow toner images in the next electronic processing unit 1C, and a full color image can be obtained.
The paper 2, on which the full color image is formed, is removed from the carrying belt 3 after passing through the electronic processing unit 1C, and is fixed in a fixing unit 14 and is output.
The above color image forming apparatus uses a so-called direct transfer system that directly transfers a toner image from a photoconductor body onto a paper. However, instead of directly transferring the single color images onto the paper, there is also an intermediate transfer system that transfers a full color image onto a paper after temporarily forming the full color image on an intermediate transfer unit from photoconductor bodies. In the intermediate transfer system, since the medium on which the color image is formed does not change its thickness and moisture absorbing property (paper changes those properties), a stable image can be obtained.
In the above color image forming apparatus, there are center distance difference among photoconductor drums, parallelization degree difference among the photoconductor drums, disposition difference among deflection mirrors, writing timing difference of exposing light to the photoconductor drums, and change of linear velocity of the photoconductor drums. Consequently, there is a problem in that images are not registered at the position where the images should be registered and displacement among colors occurs. The main reasons for this displacement are skew caused by unevenness of slant of scanning lines among colors, sub scanning registration displacement in which each image position is displaced in the sub scanning direction (carrying direction of the paper 2 by the carrying belt 3) perpendicular to the main scanning direction, sub scanning pitch irregularity, main scanning registration displacement where the writing start position and the writing end position in the main scanning direction are displaced, and magnifying power difference in which the lengths of the scanning lines among colors are different.
In the image forming apparatus shown in FIG. 1, positioning difference due to a speed change of a belt carrying unit, caused by a change of the belt thickness, eccentricity of carrying rollers, and speed irregularity of a driving motor, produces a waveform having plural frequency components as shown in FIG. 2(a). In an output image in which images are registered during the speed change of the belt carrying unit, positions of colors do not match as shown in FIG. 2(b); therefore, image quality of the output image is deteriorated, that is, displacement of colors and a color change occur.
As mentioned in the conventional technology, when marks are formed on the belt, the marks are read by an optical sensor, and the driving motor is controlled by calculating the moving speed from a time interval of read signals, and the speed irregularity and the positioning difference of the carrying belt can be reduced. As shown in FIG. 2(c), if at least low frequency components of the speed change are controlled, the displacement of colors can be reduced.
As a mark to be formed on the carrying belt, a single mark or plural marks are acceptable. However, in a case where the moving speed of the carrying belt (moving body) is detected, as shown in FIG. 3(a), when marks 26 each having a slit type pattern are formed on a carrying belt 25, which is rotated by a driving roller 22 and driven rollers 23 and 24 driven by a motor 20, with the same interval pitch, a signal whose output frequency is changed corresponding to the speed change of the carrying belt 25 can be detected by an optical sensor 27. In FIG. 3(b), the marks 26 formed in the carrying belt 25 are shown in detail, and the surfaces of the marks 26 are covered with a protecting layer 28. In this, the reference number 21 is a transmission device disposed between the motor 20 and the driving roller 22.
However, in the explanation of the conventional technology, a suitable method of forming the marks on the carrying belt is not described and problems to be solved at the time of actual usage are also not described.
For the above problems, the present inventor discloses a technology in Japanese Laid-Open Patent Application No. 2004-99248 and Japanese Priority Patent Application No. 2003-52972. In the technology, the following advantages are described by forming a surface protecting layer for marks in an endless belt carrying unit.
(1) Marks are prevented from being damaged due to contact with rollers and a cleaning blade.
(2) Lower strength caused by forming the marks is compensated for.
(3) Even when a mark made of a metal reflection film is used, a leakage current of a high voltage such as a transfer bias is prevented from being generated.
(4) When a mark protecting layer is formed, occurrence of a pitch difference between marks is prevented.
In addition, the present inventor discloses a technology in Japanese Laid-Open Patent Application No. 2004-202498.
This technology controls the speed of an intermediate transfer belt to be constant by directly detecting the surface speed of the intermediate transfer belt with the use of feedback. A reflection slit pattern is formed by applying a low heat damage process with the use of a short pulse laser beam to an aluminum deposition tape having a PET protecting layer stuck on the surface of the intermediate transfer belt. A laser process can be applied on the protecting layer so that damage to the belt is low, and a reflection type sensor can be used. That is, this technology includes materials used in this structure, laser wavelengths, and its processing method.
The following problems are shown when reflectance control of a metal material layer is executed by a laser beam process.
(1) An adhesive under the metal material layer is damaged by heat at the time of laser beam processing.
(2) It is difficult to form a pattern with high accuracy due to occurrence of enlarging the pattern part caused by thermal conduction of metal at the time of laser beam processing.
(3) Even when a protecting layer exists, there is a possibility that current leakage occurs from ends and a crack of the protecting layer.
[Patent Document 1] Japanese Laid-Open Patent Application No. 6-263281
[Patent Document 2] Japanese Laid-Open Patent Application No. 9-114348